JP-A-2009-4112 (PTL 1) and JP-A-2007-157682 (PTL 2) disclose a pump mechanism for maintaining the surrounding of an electron source in a high vacuum state (10−7 to 10−8 Pa) in a composite charged particle beam apparatus in which a focused ion beam (FIB) device for processing a sample section is combined with a scanning electron microscope (SEM) for observing the processed sample section.
Pamphlet of International Publication No. 2014/171287 (PTL 3) discloses SEM (CFE-SEM) using a cold field emission (CFE) electron source which does not always heat an electron source. The SEM performs low temperature flushing on the CFE electron source during SEM observation. The flushing means heat treatment for obtaining a clean surface by desorbing a gas molecule adsorbed on the electron source. In addition, the low temperature flushing means flushing while an extraction voltage is applied in a stage before gas is completely adsorbed to a distal end of the CFE electron source after a prescribed time limit (high brightness stabilized region) having less influence of residual gas adsorption exceeds immediately after the flushing. Since the gas is not yet completely adsorbed, it is possible to desorb the adsorbed gas at a much lower degree compared to normal flushing. In addition, since the temperature is low, even while the extraction voltage is applied, there is no build-up which changes a shape of the distal end of the electron source.